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Tanya's reborn into Stormwind during the First War and eventually grows up in Strahnbrad, being there during the start of the human campaign in Warcraft 3. Because Tanya desperately wants a cushy retirement in the capital, she does everything she can to support Arthas in his campaign against the Scourge. You get Tanya's usual Selective Obliviousness including not realizing Arthas is falling for her until her sorta proposes at the end of the story. Though the real interest is the combat along with the logistics in fighting near constant battles against a zombie horde that are by definition The Needless.
The real kicker that truly derails things isn't her constant support of Arthas or her healing, but Tanya being forced to pray to Being X in Stratholme and purifying everyone there of the plague, combined with Putress managing to find a cure due to some tips she'd given him on proper testing. Meaning Arthas never goes off the deep end, the Legion is never summoned during the Third War (which doesn't really happen anyway), and Quel'thalas is never razed. In short, except for Burning Crusade and probably Cataclysm, none of the Warcraft story occurs from the finale of the human campaign right up to the end of Legion. Or to put it another way, Tanya derails things so badly, she drives the entire Bronze Dragonflight insane. In her own weird way, Tanya manages to give all of Azeroth the happy ending they've always been denied.
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